01 - Preheat oven to 375°F. Grease a 9x13 inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Add macaroni and cook for 6 minutes. Add broccoli florets and cook both together for an additional 2 minutes, until pasta is al dente and broccoli is bright green. Drain well in a colander and set aside.
03 - In a large saucepan over medium heat, melt 4 tbsp butter. Stir in flour and cook for 2 minutes, whisking constantly, until the mixture is smooth and slightly golden.
04 - Gradually whisk in whole milk and heavy cream. Continue whisking and cook for about 5 minutes until the sauce thickens and begins to bubble.
05 - Remove saucepan from heat. Stir in shredded cheddar, Gruyère, Dijon mustard, garlic powder, onion powder, salt, and pepper until completely smooth and creamy.
06 - Add the cooked macaroni and broccoli to the cheese sauce. Mix thoroughly until evenly coated. Transfer the mixture to the prepared baking dish.
07 - In a small bowl, combine panko breadcrumbs, grated Parmesan, and melted butter. Mix until the butter is evenly distributed.
08 - Sprinkle the breadcrumb mixture evenly over the macaroni and cheese. Bake for 20-25 minutes until the sauce is bubbling around the edges and the topping is golden brown.
09 - Let the baked macaroni and cheese cool for 10 minutes before serving to allow the sauce to set slightly.